1. Field of the Invention
The present invention relates to an image recorder for directing a light beam modulated in accordance with an image signal onto a printing plate to record an image directly on the printing plate. More particularly, the invention relates to a technique for improving the precision of the positional relationship between an image recorded on a printing plate and positioning holes serving as a reference for mounting the printing plate on a plate cylinder of a printing press or positioning holes serving as a reference for mounting the printing plate on a bending machine, a puncher or the like (these positioning holes hereinafter will generically be referred to as positioning holes for printing press).
2. Description of the Background Art
Image recorders for directing a light beam modulated in accordance with an image signal onto a printing plate to directly record an image on the printing plate have conventionally been known.
An image recorder includes a punch unit and a recording drum on which a printing plate is mounted. First, the image recorder punches positioning holes serving as a reference for positioning the printing plate on the recording drum (hereinafter referred to as positioning holes for recording drum) and positioning holes for printing press at the same time in the punch unit. Thereafter, the image recorder makes the positioning holes for recording drum fit over positioning pins provided upright on the recording drum, to accurately position the printing plate on the recording drum. In this state, the image recorder directs a light beam from a recording head while rotating the recording drum at high speeds, thereby recording a two-dimensional image on the printing plate fixed on the recording drum.
In the case of recording a plurality of color component images for constituting a color image on a plurality of printing plates, the positional relationship between a color component image recorded on each printing plate and positioning holes for printing press needs to coincide precisely in the respective printing plates.
The above-described image recorder accurately positions the printing plate on the outer peripheral surface of the recording drum with reference to the positioning pins. The positioning holes for recording drum to be fitted over the positioning pins and the positioning holes for printing press are punched at the same time. Therefore, the positional relationship between these positioning holes is accurate. Further, the recording head has such a positional accuracy that an image is recorded accurately in a predetermined position on the recording drum. Therefore, an image recorded on the printing plate by the recording head and the positioning holes for printing press always have a sufficient degree of positional accuracy.
As described above, the above image recorder achieves a sufficient degree of positional accuracy between the positioning holes for printing press and a recorded image since the positioning holes for printing press are punched at the same time as the positioning holes for recording drum. However, these positioning holes are not always punched at the same time. Alternatively, the printing press may be fixed on the recording drum without punching any positioning holes for recording drum. In such cases, a sufficient degree of positional accuracy may not be ensured between a recorded image on the printing plate and the positioning holes for printing press.